The invention concerns a flexible pipe element, in particular for exhaust pipes of combustion engines of motor vehicles.
Flexible pipe elements of this type generally comprise a helical-shaped or corrugated bellows and a strip wound hose which is helically wound or which comprises several annular segments and which is coaxially adjacent to the bellows, wherein the helical winding or the annular segments of the strip wound hose have a substantially S-shaped cross-section by means of which axially neighboring elements of the helical winding or axially neighboring annular segments of the strip wound hose engage one another. They are installed as intermediate pieces in the motor vehicle exhaust pipe which is usually fixed to the vehicle floor, optionally using flexible support parts to accept movement and vibration produced by the elastically suspended engine and by changes in length due to temperature etc. and to prevent same from being passed on to neighboring structural components.
Particularly disadvantageous thereby is the limited freedom of motion of the pipe element caused by the strip wound hose. This deteriorates the decoupling properties of the pipe element when the strip wound hose has reached its maximum possible axial length and leads to transfer of load changes in the engine to the exhaust system causing unpleasant noise. The limited mobility of these pipe elements also usually requires an exhaust system which is displaceably suspended from the underbody of the motor vehicle which can accept extreme motion leading to blocking of the pipe element.
Known pipe elements (EP 0 903 528 A1, EP 0 709 558 A1, DE 296 09 293 U1) furthermore mostly comprise a metal braid disposed on the outside to protect the bellows from exterior mechanical influence and excessive lengthening. Disadvantageously, such a braid is radially extended or retracted when the pipe element becomes longer or shorter. When the braid adjoins e.g. the bordering bellows, the pipe element is additionally blocked from further axial extension and also clearly hardened against lateral and angular motions such that oscillating motions are transferred to the exhaust pipe of the motor vehicle when the pipe element is in this state. If the braid is radially separated from the neighboring bellows, it could strike the bellows and/or neighboring vehicle parts in response to transmitted engine vibrations to produce unpleasant noise.
EP 0 681 096 A1 describes a flexible pipe element comprising a bellows wherein the outer metal braid is replaced by knitted fabrics, thereby preventing the metal braid from generating disturbing striking noises. The knitted fabric can also softly support axial motion of the bellows since the mesh can be displaced in both tension and compression directions within limits which are relatively soft. However, in particular the bellows of such a pipe element may fail prematurely since the knitted fabric alone provides insufficient support. An inner pipe is preferably provided to protect the bellows and is coaxially surrounded by the bellows. This considerably limits the motion of the pipe element, in particular with respect to angular deflection.
DE 296 09 293 U1 describes a flexible pipe element comprising a bellows and a knitted fabric directly surrounding same and an outwardly disposed metal braid. This device shares the above mentioned disadvantages.
DE 198 48 451 A1 describes a flexible pipe element wherein spacers are disposed between the bellows and at least one additional element such as e.g. a strip wound hose or a metal braid to effectively damp the bellows. This pipe element has insufficient mobility for many applications.
It is the underlying purpose of the invention to propose a flexible pipe element which has increased mobility thereby preventing premature failure of the bellows, and at the same time good decoupling properties in the high and low frequency ranges, in particular for small amplitude oscillations and large amplitude load change oscillations of the engine.
This object is achieved in accordance with the invention with a flexible pipe element, in particular for exhaust pipes of combustion engines of motor vehicles with a helical or corrugated bellows, at least one strip wound hose which is helically wound or has several annular segments and which coaxially adjoins the bellows, at least one spacer locally disposed between the bellows and the strip wound hose and an outwardly disposed tube of knitted fabric, wherein the pipe element has a total stiffness of not more than 5 N/mm.
In the inventive design, the bellows is sufficiently supported by the strip wound hose in the axial and radial directions such that premature failure thereof is safely prevented. Moreover, the total maximum stiffness of the inventive pipe element of 5 N/mm advantageously permits a fixed arrangement of the exhaust system on the underbody of the motor vehicle. This is desired by the automobile industry since thermally or mechanically produced distance and angle changes between the engine and exhaust system are reliably accommodated.
The spacers locally disposed between the bellows and the strip wound hose prevent contact between the bellows and the strip wound hose due to oscillating motion of the motor vehicle and the resulting noise caused thereby as known per se from DE 198 48 451 A1. The local spacers reliably prevent the bellows and the adjacent strip wound hose from striking each other to prevent noise, in particular at resonance frequencies, due to direct collision of these components.
The tube of knitted fabric disposed on the outside protects the flexible pipe element from external influences, such as mechanical loading during assembly and hurled stones during operation, without impairing its motion. A highly flexible tube of knitted fabric of this type abuts the inside adjoining element, e.g. at least on the outer crests of the bellow corrugations and across its entire length such that generation of undesired noise due to oscillating motions is reliably prevented.
As mentioned above, the helical windings or annular segments of the strip wound hose have a substantially S-shaped cross-section by means of which axially neighboring elements of the helical winding or axially neighboring annular segments of the strip wound hose each engage one another. In a preferred embodiment, the substantially S-shaped cross-sectional profile of the helical winding or of the annular segments of the strip wound hose has a central section each end of which has an adjacent bar extending substantially in the axial direction with each bar being joined by an end bar extending towards the central section and disposed substantially in an axial direction, wherein the radial separation between the bars is larger than the radial thickness of the end bars and the axial separation between the central section and the bridges is greater than the axial length of the end bars. This embodiment forms a soft stop between each helical winding or between each of the annular segments of the strip wound hose in an axial and also radial direction to prevent distortion of the bellows due to bending, stretching or compression, etc. On the other hand, the overall length of the pipe element ensures increased mobility in the axial and radial directions since axially neighboring elements or segments of the strip wound hose can move relative to another. In particular angular motion is also possible having a maximum value which depends on the ratio between the radial separation of the bars and the radial thickness of the end bars and on the ratio between the axial separation between the central section and the bridges and the axial length of the end bars of the substantially S-shaped cross-sectional profile. Engagement of the axially neighboring elements of the helical winding or the axially neighboring annular segments of the strip wound hose with axial and radial play also guarantees a considerably increased freedom of motion with respect to axial, radial and angular movements of the pipe element compared to prior art to effect complete decoupling of small amplitude oscillations as well as engine load change oscillations of large amplitude.
The cross-sectional profile is preferably mirror symmetrical with respect to the center of the central section to permit uniform motion of the pipe element in any direction.
While the cross-sectional bars are preferably parallel in the axial direction of the pipe element, the central section is preferably a bridge disposed in a radial direction or a bridge disposed at an angle with respect to the radial direction, wherein, in the latter case, the bridge is preferably inclined in the direction of the bars disposed at the ends thereof.
Depending on the desired mobility of the flexible pipe element, the radial separation of the bars is preferably 1.1 to 3 times, in particular 1.2 to 2 times the radial thickness of the end bars and the axial separation between the central section and the bridges is preferably 1.05 to 1.9 times, in particular 1.1 to 1.8 times the axial length of the end bars.
The spacers are preferably strips and do not substantially change the outer dimensions of the pipe element due to the annular space which is in any event present between the bellows and the strip wound hose. The localized application, in particular in strips is usually sufficient to obtain the desired performance and saves substantial amounts of material compared to spacers which are disposed along the full surface. Due to their strip character, the spacers are also more flexible than a spacer disposed along the entire jacket of a component such that the pipe element is not limited in its axial, lateral and angular motion.
The spacers can be fixed to the bellows and/or the strip wound hose at one or both cylindrical connecting ends of the pipe element e.g. via cylindrical rings, or can be loosely disposed between the bellows and the strip wound hose. The spacers can be fixed, e.g. crimped at both connecting ends of the pipe element to maintain contact and prevent displacement during operation.
The strip spacers can be wound about the additional element in a straightforward fashion. They can be disposed in a radial and axial direction or in a threaded, helical fashion about the jacket of the strip wound hose adjacent to the bellows. The spacers can be disposed symmetrically about the jacket of the strip wound hose adjacent to the bellows and/or, to save material, locally at particularly vibration-sensitive regions of the pipe element, e.g. asymmetrically between the bellows and the strip wound hose.
While the spacers usually occupy a single layer, in a preferred embodiment, the spacers are crisscrossed and/or disposed in two or more layers between the bellows and the strip wound hose to provide optimum damping of the pipe element. Even when the spacers are disposed in several layers, less material is required than for a full-surface spacer in dependence on the separation between the spacer strips. Varying orientation of the strips in e.g. both an axial as well as a helical sense, can optimise damping of the pipe element.
The spacers preferably consist substantially of glass and/or mineral fibers and are preferably formed as knitted fabric, braid or fleece, in particular as fleece strips.
Alternatively or additionally, the spacers can be substantially made from metal, such as wire, metal cushion, expanded metal or sheet metal and metal foam. In this context, xe2x80x9cmetal foamxe2x80x9d is a metal mixed with a foaming agent (and which can therefore be foamed) and can be produced from a metal powder together with a foaming agent such as titanium hydride through heating and compression. If the spacer is made from sheet metal it can preferably be perforated to increase its elasticity. Spacers of plastic material are e.g. also feasible.
The frictional force exerted by the knitted tube on the bellows is preferably only 0.5 to 5 N, in particular approximately 1 to 3 N such that the flexibility of the bellows remains substantially unimpaired with the knitted tube always seating on the bellows. The frictional force can be adjusted e.g. in a simple fashion via the inner diameter of the knitted tube or the normal force it exerts on the bellows.
To reliably fix the knitted tube, it is preferably fixed to the connecting ends of the pipe element, wherein it can preferably be held by cylindrical rings. The cylindrical rings also preferably serve for fixing the bellows, the strip wound hose and optionally the spacers. Fixing of the knitted tube has various advantages not only with regard to the unravelling of the ends of the knitted fabric where the tube is usually cut-off but also to assure the protective function of the knitted tube along the entire length of the bellows as well as providing a soft-tension stop to protect the bellows during stretching.
While the knitted tube preferably consists of metal wire, it can also be produced from temperature-resistant fiber material. The knitted fabric can thereby have one or more threads. Construction of the tube with one or more threads can increase its flexibility and reduce its rigidity.
The meshes of knitted fabric preferably extend substantially in the axial direction of the flexible pipe element. This assures minimal influence on the mobility of the pipe element, in particular in the axial direction since the meshes can be freely displaced towards one another in the axial direction during compression.
The bellows of the inventive pipe element preferably has a rigidity of approximately 2 to 5 N/mm.
Although the strip wound hose can, in principle, be disposed outside the bellows to, in this case, be directly surrounded by the knitted tube, in preferred embodiments the strip wound hose is disposed inside the bellows.